1. Field of the Invention
This invention relates generally to luminance signal forming circuitry and, more particularly, to a luminance signal forming circuit applicable to a color television camera using complementary color filters.
2. Description of the Background
As is well known, complementary color filters are used to block a selected color component of the primary colors, that is, red (R), green (G), and blue (B). A yellow (Y.sub.E) complementary filter passes R and G components therethrough, a cyan (C.sub.Y) complementary filter passes G and B components therethrough, a magenta (M.sub.G) complementary filter passes R and B components therethrough, and a white (W) complementary filter passes all R, G, and B primary color signals therethrough.
Thus, two or more primary color components (R and G, G and B, R and B, or R, G, and B) can pass through a single filter when these complementary color filters are used in a color television camera. Therefore, by reducing the number of filters in the light path, the quantity of light of the primary color signals incident on the image pickup device, such as an image pickup tube or a solid-state image pickup device, is increased. Accordingly, even when the image comprises a dark object a video signal output having a high level can be obtained from a camera using complementary filters.
In a conventional complementary color television camera of this kind, the video signal produced by the image pickup device is transmitted as a luminance signal through a low-pass filter (LPF) that functions as a simple composing circuit for the outputs from the camera. This low-pass filter may be through of as a separate luminance signal system that is provided to prevent the complementary color filters from losing their advantage, that is, high output from limited image brightness, and also to prevent noise from mixing with the luminance signal.
When a luminance signal is generated by simply composing or adding video output signals through a low-pass filter, as in the above-described proposed complementary color television camera, it is difficult to satisfy all three of the following conditions because of the inherent characteristics of the luminance signal.
In regard to the first condition, in general the ratio of the primary color signals, that is, the R, G, and B signals included in the luminance signal Y of the color television camera, is determined by the individual standard television scheme. For example, in the NTSC scheme, the ratio of R, G, and B primary color signals is standardized at 0.3:0.59:0.11. This means that the luminance (Y) can be expressed as follows: EQU Y=0.3R+0.59G+0.11B (1)
In practice, complementary color filters are designed to generate a luminance signal having a ratio close to that of the above and the luminance reproducibility or fidelity of the filtesr is optimized as the luminance signal approaches the exact ratio of equation (1).
As the second condition, in the complementary color television camera, the R, G, and B primary color signals are obtained by matrixing a plurality of complementary color signals obtained from the image pickup device, however, if there are variations in the sensitivity of the plurality of image pickup devices with respect to the complementary color signals, then folded distortion may be mixed with the luminance signal. Folded distortion is the so-called folding noise that is known in the field of signal sampling.
For example, in a complementary color television camera having the Y.sub.E and C.sub.Y complementary color filters, as well as the white color filter (W) for transmitting the R, G, and B primary color signals therethrough, if the B, R, and G primary color signals are obtained by performing operations represented by the following equations on the basis of the W, Y.sub.E, and C.sub.Y complementary color signals obtained from an image pickup device, the W, Y.sub.E, and C.sub.Y complementary color signals are sampled during one pixel period 1T, as shown in FIG. 1. This is also represented as follows: EQU W-Y.sub.E =(R+G+B)-(R+G)=B (2) EQU W-C.sub.Y =(R+G+B)-(B+G)=R (3) EQU Y.sub.E +C.sub.Y -W=(R+G)+(B+G)-(R+G+B)=G (4)
In this case, each of the periods of the W, Y.sub.E, and C.sub.Y complementary color signals is three times the sampling frequency used for sequentially sampling the W, Y.sub.E, and C.sub.Y complementary color signals. More specifically, W, Y.sub.E, and C.sub.Y are sequentially sampled by a sampling signal having a first frequency, so that each obtained W. Y.sub.E, and C.sub.Y output signal has a frequency equal to one-third that of the first sampling frequency. Thus, each of the obtained outputs W, Y.sub.E, and C.sub.Y has a period that is three times longer than that of the sampling signal having the first frequency.
Accordingly, as shown in FIG. 2, in the signal components of the video output signal from the image pickup device, the W, Y.sub.E, and C.sub.Y complementary color signal components are produced at a position corresponding to a sampling frequency f.sub.s on a spatial frequency axis f, and the W, Y.sub.E, and C.sub.Y complementary color signal components having mutual respective phase differences of 120.degree. are also produced at positions of f.sub.s /3 and 2f.sub.s /3, respectively. Further lower and upper sideband modulation signal components LB1 and UB1, and LB2 and UB2 having the sampling positions 1/3f.sub.s and 2/3f.sub.s as the centers are also produced.
The lower sideband modulation signal LB1 of the W, Y.sub.e and C.sub.Y complementary color signal components having a frequency of f.sub.s /3 may be folded back so as to reach the baseband luminance signal component Y and can become mixed in a comparatively high-frequency range of the luminance signal component Y. This folding back and mixing causes degradation of the image quality, for example, fringe patterns will appear in a reproduced image, in accordance with the so-called folded distortion.
In regard to the third condition, in a conventional complementary color television camera suitable signal processing is performed so that the R, G, and B primary color signals are formed by performing the above-described conversion operation using equations (2) through (4)and the W, Y.sub.E, and C.sub.Y complementary color signals, and the luminance signal Y is formed by mixing the primary color signals at the ratio set forth in equation (2). Nevertheless, when such two operations are performed, noise mixing inevitably occurs and, even more problematical, this noise may be mixed over the whole frequency range of the luminance signal Y.
The level balance of the complementary color signals and the luminance reproducibility of the luminance signal Y are determined by the spectral sensitivity of each of the plurality of elements constituting the image pickup device and by color coding during conversion of the complementary color signals into the primary color signals.
As described above, in the conventional complementary color television camera the complementary color signals obtained from the image pickup device are processed simultaneously regardless of their respective frequencies. Therefore, when characteristics of the luminance signal are desired to be changed to meet a specific purpose, a luminance signal having such characteristics cannot be formed because of poor parameter flexbility.
In the conventional complementary color television camera it has been found to be difficult to maintain good luminance reproducibility, while effectively restricting folded signals and noise, by controlling the level balance of the complementary color signals.